Energy Independence and Electricity Storage

September 1, 2012 by  
Filed under Green Energy News

willem Post said:

MK.

DISPATCH VALUE, VARIABILITY AND INTERMITTENCY OF WIND ENERGY

Dispatch Value: Wind energy is significantly different from conventional gas, coal, nuclear and hydro energy; just ask any grid operator with significant wind energy on his grid. The latter are controllable and dispatchable on short notice,  whereas wind energy is a product of weather-dependent, variable wind speeds, i.e., its supply is unpredictable and uncontrollable, and therefore, it has zero-dispatch value to a grid operator. 

Note: Wind energy supply can be controlled/curtailed by feathering the blades and stopping the rotors which owners object to as it reduces their production and their federal PTC payments. In some grid areas, owners are compensated for not producing.

A grid operator needs to have available an adequate mix of generating capacity to serve peak demands for long-term planning purposes. The mix varies from grid to grid. Wind turbine systems have a capacity value in this mix. 

Example: For summer peak capacity planning, ERCOT counts 8.7 percent of wind turbine rated capacity as dependable capacity at peak demand, in accordance with ERCOT’s stakeholder-adopted methodology. According to ERCOT, the capacity value is a statistical concept created for generator planning purposes. It is based on multi-year averages of wind energy generation at key peak demand periods. 

http://www.ercot.com/news/press_releases/show/381

ERCOT’s capacity planning value of 8.7% does not mean the ENERGY of 8.7% of wind turbine rated capacity would be available at any specified “time-ahead” period. Because of the randomness of wind speeds, no one can accurately predict available wind energy at any future time. Hence, it’s not available “on-demand”, i.e., not dispatchable.

Variability: Because wind energy increases by the cube of the wind speed, any change in wind speed creates significant surges and ebbs of wind energy. If such energy were fed into the grid, it would create chaos. 

Thus, wind energy cannot stand on its own, has no value on its own, is completely useless, unless the grid has an adequate capacity of quick-ramping gas turbines and/or hydro plants that are required to inefficiently operate at part-load to be able to ramp up when wind energy ebbs and ramp down when it surges, which happens at least 100 times per day, to maintain grid frequency and voltage within required limits. 

If a grid does not have adequate capacity of such ramping plants, it either must acquire it, or connect to grids that do have it and do not need it for their own variable wind and solar energy.

Many grids, including Germany’s four grids, the Bonneville Power Authority, Texas, Colorado, Germany, Spain, etc., do not have a sufficient capacity of such quick-ramping generators. As wind energy on the grids increases, the grid operators are unable to balance the wind energy and need to  transfer it to neighboring grids for balancing, if possible, and/or implement curtailments.

Example: German wind power output, peaked at about 12,000 MW on July 24, 2011, four days later the peak was 315 MW; Germany’s wind turbines are located mostly in Northern Germany. 

Intermittency: Wind energy generation usually is minimal during summer (it is almost non-existent in New England), moderate during spring and fall, and maximal during winter. Almost all the time it is maximal at night. 

About 10-15 percent of the hours of a year wind energy is near zero, because wind speeds are insufficient (less than 7.5 mph) to turn the rotors, or too great for safety; about 25-30 percent of the hours of a year in New England. During these hours, wind turbines draw energy FROM the grid, and also during hours with slowly turning rotors when parasitic energy exceeds the generated energy. Rotors are often kept turning with grid energy to prevent the rotor shaft from “taking a set”.

 

Comments are closed.